I had quite a bit of steel powder coated but not aluminum, although the process would be similar. With steel, I have it slag blasted (not sand, can lead to coating failure) and then drop it off the the PC shop. The PC needs to have a clean semi-rough surface to properly adhere to. All paint, clearcoat, rust, dirt. etc. must be removed prior.
Hope this helps.
I suggest an epoxy powder-primer and a TGIC-polyester top coat....
be advised that the success of the powercoat is totally up to your coater if they use good materials, and know how to dial in line speed and bake temp for your parts....YMMV
I powder coated just about everything on my dragbike, including the wheels...bug strikes at 175 mph used to damage the paint...and powder coat is impervious to just about any kind of fuel or oil...just be sure the coater properly masks or plugs anything that you dont want the coating in or on.